This invention relates generally to a reinforcing support structure for an automotive vehicle body, and more particularly to stiffeners integrated with the sheet body structure.
Stiffened sheet structures play a vital role in today""s automotive body construction. To a significant degree these stiffened sheet structures serve to define the aesthetic shape and style as well as important safety elements of the vehicle. These structures have long provided an effective medium for addressing some of the unique and demanding aesthetic and structural requirements of this industry. While many technical advances have been made, improved stiffeners for automotive body sheet structures are still needed.
Over the past several years government-mandated vehicle crash test requirements as well as government and industry fuel economy and safety goals have driven the automotive industry toward lighter, stronger, more energy absorbing, and tailorable stiffeners for integration with automotive sheet material bodies, including those used in cars, trucks, minivans, and sport utility vehicles, in order to maximize structural efficiencies related to these goals. Viable candidate stiffener concepts must be capable of addressing not only performance goals, but also economies related to processing, forming, structural integration/configuration, tooling, and assembly. The automotive industry has thus embarked on an exhaustive search for simple, enabling stiffener technologies that might fulfill these requirements.
Generally speaking, even the most favored conventional stiffener technologies have satisfied only some of the combined goals, in spite of the significant advantages made possible by using the latest and most advanced technologies now available in structural simulation and stress analysis as well as in optimization computer software. These efforts have sometimes made use of very high and ultrahigh strength materials including specially developed steel alloys and extrudable aluminum alloys, as well as ceramic fiber reinforced plastics.
The most commonly resulting trend has been for weight saving and strength goals to be heroically accomplished in tandem with significant penalties in the areas of tooling, material processing, fabrication, and assembly costs. At times, form and style have yielded to functional goals. The total net cost savings has sometimes been marginal. This is because of the complex cross-sections and interfaces that are sometimes created as mass is redistributed using an increasingly intricate and localized level of control over the cross section of each structural component. As a result, fabrication, integration, complexity and space claim issues have typically risen to join a growing list of challenges.
Some of the favored cross-sections have included modified tubes, hat-shaped cross-sections, and C-channels of various types. Each of these shapes offers specific challenges in the area of interfaces and joints. Some of these shapes have been formed under very high fluid pressures that may themselves have presented new safety and training challenges in their implementation in the workplace. One common scenario in these cases has been that as design ratios of cross-sectional dimensions such as outer diameter-to-thickness or depth-to-thickness ratios exceed the range of about 50, both closed and open sections may have entered a range of relatively high sensitivity to local wall thinning during fabrication, as well as sectional buckling and reduced bending rupture resistance in service.
Furthermore, the use of thinner material in traditional open-section stiffener configurations makes these stiffened sections more susceptible to edge stress concentrations that are characteristic of open sections-especially under bending and compression loads. This is because conventional thin open sections commonly have a xe2x80x9cblade edgexe2x80x9d. This edge is very susceptible to imperfections in the sheet material along this edge as well as to damage during manufacture, shipping/handling and installation. These imperfections along the blade edge become stress concentration points or focal points at which failure of the stiffener can initiate. A more detailed description of this failure initiation follows.
Even the most perfect, smooth edge of the conventional stiffener may experience a very localized point of high stress gradient due to the characteristic edge stress concentration associated with open sections under bending loads. Thus, initiation of an edge xe2x80x9cbulgexe2x80x9d or xe2x80x9ccrimpxe2x80x9d on a perfect smooth edge is nothing more than the creation of an edge imperfection that is large enough to grow or xe2x80x9cpropagatexe2x80x9d easily. It is significant that this stress concentration may be made worse by the presence of any relatively small local edge imperfections, even those on the order of size of the thickness of the stiffener material itself.
These imperfections near the edge can be in the form of edge notches, waviness (in-plane or out-of-plane), local thickness variations, local residual stress variations, or variations in material yield strength. Where multiple imperfections occur together, they may all compound together to further increase the stress concentration effect, and thus lower the load level at which failure is initiated. Thus, the existence of any edge imperfections in a conventional open section stiffener has the effect of enhancing an already established process of failure initiation.
Local complexity in the structural cross sectional shape of thin conventional stiffeners can further degrade structural stiffness and buckling resistance. Buckling is an instability in a part of the stiffener associated with local compressive or shear stresses. Buckling can precipitate section failure of the stiffener. This in turn can cause a stress concentration in adjacent structures that can lead to failure of a larger section. This effect is of particular concern in the evaluation of crash worthiness of automotive bodies, because such failures may be less uniform or predictable, thus making them less desirable from an occupant safety standpoint. Such unstable structures typically do not absorb sufficient crash energy or resist crash forces effectively enough to consistently meet safety performance goals without adding significant additional mass and further complexity to the overall design.
Moreover, some thinner conventional stiffeners can experience xe2x80x9crollingxe2x80x9d when placed under load. Rolling is when the shear stresses within the stiffener result in a net torque about the centroid of the thin walled cross-section, thus causing the cross-section to twist, possibly making the stiffener unstable. Another cause of rolling is related to the curvature of the stiffener itself, after it has been formed to the local contour of the vehicle. Some designers have increased the cross sectional length of the open-section member flanges having free edges while attempting to solve the rolling problem but were met with only marginal improvement. This is because the increased flange length has the simultaneous effect of increasing the distance from the centroid to the shear center of the stiffener. Also, increasing the cross-sectional flange length sometimes causes difficulty in accessing the interior of the section during assembly or other operations and has typically raised additional space claim issues.
Yet another problem facing thinner conventional structural stiffeners is that of fastening or joining relatively thick sections to sections that are relatively less thick, or relatively stiff sections to sections that are relatively less stiff near the joint or interface. This can result in a local stress concentration in the region of joining. These stress concentrations may significantly weaken the joints or interfaces associated with conventional stiffeners.
Computer optimization codes often help to improve the design of conventional stiffeners. But they may not accurately represent the degradation in practical performance and increased sensitivity to geometric and material imperfections that has brought largely empirical guidelines into widespread acceptance over the years. Moreover, as conventional stiffening cross-sections are made thinner, damage tolerance may rapidly become an even greater concern. Load paths and local fastener stresses become more difficult to evaluate. Even minor repair is sometimes out of the question due to special welding or joining techniques that do not lend themselves to body shop environments. Because of ever increasing safety and performance standards, there is clearly an established need for a new and innovative automotive body structural stiffening system. The new system should combine traditional versatility and simplicity with design flexibility and tailorable structural efficiency to achieve both weight saving and crash worthiness objectives, while not casting a shadow on form and style. Such a system should specifically address and overcome the significant limitations of conventional stiffener shapes, without resorting to extreme measures, for example in fabrication or joining approaches. Such a system should largely follow existing intuitive conventions for joining and forming, handling and processing, without presenting additional obstacles to workers or to the environment.
The present invention alleviates and overcomes the above-mentioned problems and shortcomings of the present state of the art through a novel stiffener for automotive body sheet structures. The novelty and uniqueness of this invention is that it: 1) is made of thinner material to reduce the in-plane interface stresses found in the sheet interface areas, 2) resists deflection adequately to meet new higher structural stiffness requirements, 3) is resistant to buckling and rolling, 4) effectively addresses edge stress concentrations by modifying the blade edge to an area of relatively low stress, 5) provides a special interfacing flange capability that results in stronger and more crash resistant joints, 6) is synergistic with the use of thinner and stronger materials, and, 7) can be manufactured cost-effectively by using conventional forming methods such as roll forming and stretch forming.
This novel invention may be described as a substantially reconfigured or stabilized J-stiffener having a specially configured interface and mounting flange capability. It should be noted here that due to their extreme susceptibility to rolling, conventional J-stiffeners are seldom used in automotive body applications. The unexpectedly strong synergisms of the unique characteristics found in the stabilized J-stiffener not only address the above problems, but simultaneously obtain significant material savings. More particularly the synergisms may be described as follows.
The instant invention has substantially redistributed material at critical locations as compared with conventional stiffener or structural member configurations. This material redistribution has the effect of altering considerably the behavior of the stiffener as compared with conventional J-stiffeners and other stiffener configurations.
The material redistribution required to accomplish these collaborative effects is accomplished by having specifically placed free edge portions, which are turned as folds to define tubular beads or curls along the free edges. Moreover it is not just the presence of the tubular bead or curl that enables the substantial level of synergism, but the discovery of specific ratios of curl diameter to other stiffener dimensions that maximize these synergisms even to the extent of obtaining significant weight savings.
Three main sets of synergisms combine to make the present invention successful. The first set of synergisms is directly related to the ratio of the diameter of the curl to the stiffener section flange length and web length. Each tubular bead has a cross-sectional dimension which when combined in specific ratios with other stiffener dimensions substantially maximizes the moment of inertia of the overall section about the horizontal and vertical axes with a minimal use of material. Moreover, the tubular bead size specified by these same ratios has the effect of altering the characteristic failure mode normally associated with the free edge stress concentration of conventional stiffeners as described above. Not only does this substantially enhance in-service performance, but it also increases the formability of the stiffener by enabling the section to respond more uniformly during three-dimensional fabrication operations such as roll forming, pressing, or stretch-forming. Finally, the cross-sectional dimensions of the tubular beads of the stabilized J-stiffener make this novel stiffener less sensitive to edge imperfections and damage because the blade edge has now been placed in a position of relatively benign stress levels so that imperfections or damage to the tube or edge region have to be on the order of size of the diameter of the curl in order to have significant detrimental effect to the stiffener section.
Having established the above ratios, a second set of synergisms was discovered by directly combining the above with specific ratios of the stiffener""s cross-sectional web dimension to cross-sectional flange dimension. The compounding effect of the first set of synergisms with this additional set of ratios makes the stabilized J-stiffener more resistant to rolling and buckling and thus avoids the problems that plague deeper conventional automotive body stiffeners using thinner gauge material. Additionally, these compounding synergisms make this stiffener unique in that stresses are now more evenly distributed in the flanges, thus making the stiffener more stable and less sensitive to dimensional imperfections. Because of these cooperative effects, the stabilized J-stiffener demonstrates its uniqueness and efficiency in using thinner gauge material to reduce in-plane stresses found in the fastener, interface, or joint areas, thus allowing the other automotive body components and the stiffener to work together as a cohesive system instead of as individual components.
The third set of synergisms relates to intermediate interface flanges of the stabilized J-stiffener. These intermediate flanges extend the capability of the J-stiffener in the following way. They permit the stabilized J-stiffener to be integrated more easily and successfully with complex arrangements of structural members, while substantially maintaining the benefits of the present invention. These intermediate interface flanges are formed according to precise and specific ranges of the angle defined between adjacent flanges, and according to specific ranges of radius-to-thickness ratios of the bend region between these flanges. The result is that the adjacent flanges to an interface flange work together to provide a significantly stabilized and strengthened interface region that lends itself to a variety of fastening or joining approaches, without substantially compromising the performance of the stiffener. In addition, when fasteners or local joints are used, an innovative barrier to crack propagation and fracture in the joint region is created. Because this innovation allows the stabilized J-stiffener to effectively address the problems of fracture and in-plane stresses at interfaces, increased performance is made possible at a larger level without significantly adding complexity and tooling costs.
Some of the mechanisms and technical advantages of better stabilization against crack growth that may originate near a bolt hole, a weld, or at other interface or joint locations along the length of the stiffener may be further described as follows. Enhanced fracture resistance is enabled by the combined effect of radius and angle between two adjacent flanges, and is still further enhanced in a compounding manner by the appropriate choice of radius to thickness ratio. This ratio serves to accomplish the dual role, first of maximizing the amount of strain hardening in the radius region which itself serves as a barrier to crack growth, and second, of emphasizing the stiffening and constraining role of one flange with respect to the other which serves as an additional crack barrier. Thus, within prescribed ranges, these variables may be changed along the length of the stiffener along with flange widths to accomplish various design goals, while substantially retaining the benefits of the teachings of the present invention. As previously stated, the dual stabilization that is achieved has a compounding effect against the growth of cracks that may originate near interfaces, joints, or welds or at other locations along the length of the stiffener. It results in a stronger, more stable stiffener that is more predictable and better able to resist damage resulting from impact for example by another vehicle. The combined effect is so significant that for some applications the strength of this unique interface flange joint may be increased by as much as a factor of about three (3).
When compared with conventional stiffeners on the market today, the stabilized J-stiffener of the present invention uses substantially thinner material while obtaining better structural performance. Thus, even though additional slit width (width of the sheet of material from which the stiffener is made) is required to reposition needed material, the use of thinner gauge material more than offsets the additional slit width, bringing overall material savings as high as 20 percent in some instances. This innovation in system configuration also represents a substantial cost savings for the manufacturer, since material cost is a significant portion of total manufacturing costs for automotive body hardware. Thus, this unique and novel stiffener is very cost effective.
For manufacturing process cost efficiency, the tubular bead is preferably an open-section bead, meaning that the sheet material is formed in an almost complete bend or curl, but the curl need not be closed at its outer edge, such as by welding. A closed section tubular bead would work equally well, at a slightly higher manufacturing cost. This edge feature is discussed in more detail in the following paragraph. The edge-flange section curl and the trough curl are folded tubular features, preferably open-sections, that are made by shaping the free edges or edge marginal portions of the stiffener cross-sections into a generally elliptical, preferably circular, cross-sectional shape. As used herein, a circular cross-section is an embodiment of an elliptical crosssection and is covered by the term xe2x80x9celliptical cross-sectionxe2x80x9d or xe2x80x9celliptical cross-sectional shapexe2x80x9d. The term xe2x80x9ccharacteristic diameterxe2x80x9d refers to a constant diameter in the case of a circle, while other elliptical shapes will have major and minor axes or diameters, with the minor axis or diameter being the xe2x80x9ccharacteristic diameterxe2x80x9d.
Even though some configurations of a slightly non-circular elliptical shape may be more desirable in some applications, the circular cross section embodiment is generally preferable, because it is simpler to manufacture, while still achieving the desired benefits to a significant degree.
It is important to contrast the edge curl approach against other possible edge treatment approaches by noting that the dimensional order of size effect related to imperfections or damages described above for the curl can not be achieved by simply folding the edge over, either once or multiple times, because in this case the characteristic dimension will be defined by the fold edge diameter and not by the length of overlap of the fold. This is because the overlap direction is transverse to the edge and quickly moves out of the peak stress region, and because the edge fold diameter defines the maximum distance over which the edge stresses may be effectively spread.
The elliptical or circular open-section tubular shape or xe2x80x9cedge curlxe2x80x9d is contrasted to tubular sections of rectangular cross-sectional shapes, including folded edges, and to open-section tubular shapes of softened corner polygon cross-sectional shapes in that the characteristic diameter will be defined in each of these other cases by the fold diameter or by the softened corner diameter nearest to the stiffener edge, as opposed to the overall diameter of the edge curl section. It may be noted that in this context a polygon cross-section with very softened corners is in effect an imperfect ellipse or circle.
In some instances, quasi-elliptical or quasi-circular cross-sections, teardrops, imperfect ellipses, and imperfect circles, in the form of polygons or rectangular cross-sections with some rounded regions may function adequately, but may also be more difficult to manufacture and may be less effective than a generally circular curl. Including local offsets or adding material locally such as by bonding or welding strips of material or high strength fibers or wires are additional examples. Yet other examples include local modifications to the material such as by heat, electromagnetic, chemical, or deformation treatment of the tubular bead cross-section or of adjacent regions. In spite of the potential for additional fabrication costs, some of the above variations may at times be desirable for example in local regions where the designer desires local regions of modified cross-sectional shape for space claim, interfacing, or joining reasons, or in special cases where such features may be combined with other features such as notches, folds, or hole patterns near the edge region in order to induce a prescribed response of the stiffener, such as in addressing crash energy absorption design-related goals. In some applications the curl may be formed by turning the edges through an arc of up to 360 degrees, 720 degrees, or even more, so that the edge loops over one or more times on itself, in order to concentrate mass locally or to address other design objectives. In these cases manufacturing economy and complexity are also considerations.
Some of the substantial advantages made possible through the teachings of the present invention may be summarized as follows. They include the synergistic effect of the stabilized J-stiffener""s material efficiency in obtaining the desired bending rigidity or moment of inertia, the alteration of the characteristic failure mode, the reduction in sensitivity to edge imperfections and damage, higher fracture resistance and more stable stresses in the regions of joints and interfaces, resistance to buckling and rolling, as well as the ability to spread stresses more uniformly. These features offer the same degree of compounding advantage as the conventional stiffener""s compounding disadvantage of low resistance to buckling and rolling combined with sensitivity to relatively small edge or dimensional imperfections. Accordingly, it can now be appreciated by those versed in this art, that the novel stabilized J-stiffener of the instant invention provides a solution to some important problems that the stiffened sheet material automotive body art has sought to overcome.
In summary, the stabilized J-stiffeners of the present invention, having specially configured interface capabilities including mounting flanges that are uniquely designed to be compatible with substantially all types of standard sheet material automotive body structures, are thereby significantly capable of lowering costs and reducing the number of stiffener types that designers must consider to achieve their objectives. These novel stiffeners thus permit more stringent structural and safety requirements to be addressed. Since they are quite adaptable, they often permit this to be done without major modification of other hardware.
The description of the present invention may incorporate dimensions that are representative of the dimensions which will be appropriate for some common automotive applications. Recitation of these dimensions is not intended to be limiting, except to the extent that the dimensions reflect relative ratios between the sizes of various elements of the invention, as will be explained where appropriate.
It is an object of the invention to provide stiffeners for an automotive sheet structure and particularly to a provide and effective stiffener for an automotive side sheet.
It is a further object to provide an economical stiffener that is capable of extending in a longitudinal direction for substantially the entire length of an automotive vehicle.
Another object is the provision of a stiffener for an automotive side sheet structure in which the stiffener has a pair of flanges with free edges and a tubular bead extending along the free edge of each flange for reinforcing the stiffener.